Sensor interfaces like Short PWM Code (SPC) and Single Edge Nibble Transmission (SENT) use a Pulse Width Modulation (PWM) encoding for the transmission of 4 bit nibbles. A checksum is attached to the message to be transmitted in order to protect the message. The protection is usually a 4 bit Cyclic Redundancy Check (CRC).
Standard SENT or SPC use a N-bit CRC over all the bits of the message. Some protocols enhance the length of the CRC to better protect the message to be transmitted. However, this reduces the possible payload for a given channel. The Signal to Noise Ratio (SNR) of the Most Significant Bit (MSB) is very high due to the PWM. The probability of a MSB change is very small. If an error occurs in the MSB of the 4 bit nibble, then the PWM signal changes from a long pulse to a short pulse or vice versa. Thus, the MSB error is easily detectable using timing evaluation.